1. Field of the Invention
The present invention relates to a two-dimensional solid-state image pickup device.
2. Description of the Related Art
Since a design rule of a semiconductor manufacturing process has been improved and the number of pixels in a solid-state image pickup device has been increased, miniaturization of the size of a pixel has been progressed. In a solid-state image pickup device, a plurality of pixel regions are arranged in a two-dimensional matrix in X and Y directions. Each pixel region includes at least a light-receiving element. Now, the size of a single pixel region is substantially twice a visible wavelength. In such a solid-state image pickup device, a single pixel region has a smaller physical size than an Airy disk, which is a diffraction limit of a lens optical system.
As a light-condensing element which efficiently guides light to a pixel region, an on-chip micro convex lens is typically used. Unfortunately, the on-chip micro convex lens has a low light-condensing efficiency for light being obliquely incident on the light-condensing element. Hence, the relative positional relationship between the light-receiving element and the on-chip micro convex lens in a pixel region at a center portion of the solid-state image pickup device is changed from that in a pixel region at a peripheral portion of the solid-state image pickup device. In particular, the position at which an optical axis of the on-chip micro convex lens passes through the light-receiving element is changed (for example, see Japanese Unexamined Patent Application Publication No. 63-39009).
When the size of the pixel region is decreased, the on-chip micro convex lens should have a processing accuracy at the sub-micrometric scale. However, it is extremely difficult to highly accurately fabricate an on-chip micro convex lens having a small focal length and a large curvature by an existing method, i.e., by melting a resist material through thermal reflowing, and using a surface tension of the resist material to form the on-chip micro convex lens. Since the forming method uses the surface tension, it is difficult to form a structure other than a spherical structure.
A light-condensing element formed of a sub-wavelength lens (SWLL) having a periodic structure at a physical scale, which is smaller than a wavelength of an electromagnetic wave (for example, visible light) incident on a light-receiving element, attracts an attention as a technique which can efficiently condense light even with a very small pixel region and which can be freely designed (for example, see Japanese Unexamined Patent Application Publication No. 2001-108812). Herein, a sub-wavelength is a region equivalent to or smaller than a wavelength of an electromagnetic wave as a subject. A sub-wavelength lens has a high light-condensing characteristic within a large incidence angle range. Thusly, the sub-wavelength lens has an excellent characteristic for obliquely incident light. For example, a sub-wavelength lens disclosed in Japanese Unexamined Patent Application Publication No. 2006-351972 has a concentric structure in each of pixel regions. When the sub-wavelength lens is applied to a solid-state image pickup device which is suitable for use with visible light, a technique has to be provided for highly accurately fabricating a structure at a smaller space scale than a wavelength of visible light. For example, the wavelength of visible light may be in a range of from about 400 to 700 nm. Thus, it is necessary to form a structure with at least a size in a range of from 100 to 200 nm.